Machinery's Handbook, 31st Edition
SOLIDIFICATION AND COOLING OF METALS
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Fig. 4. Cooling Curve for a Poured Pure Metal During Casting
Solidification of Pure Metal.—Pure metal solidifies at constant temperature. It has a clearly defined melting (or freezing) point. Fig. 4 shows the cooling curve for a poured pure metal during casting. After the temperature of the molten pure metal drops to its freezing point, the tempera- ture remains constant while the latent heat of fusion is given off. The solidification front (solid–liquid interface) moves through the molten metal, solidifying from the mold walls toward the center. At the mold walls, the molten metal cools rapidly and first produces a solidified shell of fine, approximately equal-dimension-in-all-direction (equiaxed) grains. Starting with these grains, the grains grow upon themselves, in the opposite direction of the heat transfer out through the mold. Those with favorable orientations—that is, away from the surface of the mold—are columnar in shape; as the driving force of the heat transfer is re- duced (farther from the mold walls), the grains again become equiaxed, as well as coarse. Those grains that have substantially different orientations are blocked from further growth. When the heat is segregated rapidly during solidification, it leads to fine structures due to a decrease in diffusion rates. Solidification of Alloys.—Solidification begins when the temperature of the alloy drops below the point of liquidus, and it is complete when it reaches solidus. A phase diagram and a cooling curve for alloys during casting are shown in Fig. 5.
Fig. 5. Phase Diagram and Cooling Curve for Alloy Composition During Casting
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